Flame detectors are used in a myriad of systems and devices. For example, many gas turbines, including both aircraft turbine engines and industrial gas turbines, include a flame detector to detect flame ignition within the combustor, and to monitor the presence and stability of the flame once it has ignited. During engine startup the flame detector provides a signal to, for example, the engine controller indicating that the fuel being supplied to the combustor has ignited. During engine operation, the flame detector monitors the continued presence and stability of the flame to detect and/or prevent adverse engine and combustor system operations, such as a flashback condition, a flameout, or various other combustion anomalies.
A relatively wide variety of flame detectors have been, and continue to be, developed that are implemented using myriad technologies. For example, phototubes, thermocouples, ionization sensors, photodiodes, and various semiconductor devices, just to name a few technologies, have been used to implement flame detectors. No matter the specific implementation, most flame sensors are supplied with a source of electrical excitation power during operation. In some instances, the power is supplied via a transformer that couples an alternating current (AC) excitation signal to the flame sensor. In at least one particular type of flame detector, the AC excitation signal is supplied, via the transformer, as a trapezoidal waveform.
Although the flame detector that is supplied with a trapezoidal waveform AC excitation signal operates safely and is generally reliable, it does suffer certain drawbacks. Specifically, the transformer that is used to couple the trapezoidal waveform AC excitation signal to the detector may have some stored residual magnetism. Thus, when the flame detector is energized, and the trapezoidal waveform AC excitation signal is first supplied to the transformer primary winding, the flux generated by the excitation signal can combine with the residual magnetism and cause the transformer to magnetically saturate. This, in turn, can cause excess current to be drawn from the trapezoidal waveform AC excitation signal source.
Hence, there is a need for a circuit and method of reducing the amount of current that is drawn from a trapezoidal waveform AC excitation signal source when a flame detector, or other device, is being energized. The present invention addresses at least this need.